The present invention pertains to superimposing additional information, such as copy control information, onto image information in an inconspicuous manner.
With the widespread use of digitally stored material in recent years, such as on Internet databases and portable storage media as digital video discs (DVDs), copyright infringement of such material has become problematic. One method that has been commonly used to mitigate this problem is to store copy control information as additional information along with the digital content on the storage medium. The copy control information typically consists of a code containing two or more bits indicating a copy generation number of the storage medium and/or whether copying of the storage medium contents is permitted. In any event, it is desirable to prevent a potential pirate from knowing and/or altering the additional information.
As a method for superimposing additional information with digitally stored image information, a digital watermark process has attracted attention from the point of view of difficulty of falsification of the additional information. See, for example, the February 24 issue of xe2x80x9cNikkei Electronicsxe2x80x9d magazine, 1997, No. 683, which describes contemporary digital watermark techniques and their advantages.
The digital watermark process is a process for embedding additional information as noise directly onto image data in a manner whereby the additional information is largely imperceptible to a viewer. The additional information embedded in the image data by such a digital watermark process is not easily removed from the image data, nor can it be easily altered. Advantageously, even after a filtering or data compression process is performed on the image data, with the proper decoding system it is possible to detect and utilize the additional information embedded therein.
Additional information that is superimposed onto an image signal by a digital watermark process is not removed from the image signals when the corresponding image is displayed on a display. Consequently, to prevent degradation of the reproduced image, the additional information should be superimposed onto the image signal at a very low level. For many digital image reproduction systems, however, some degradation of image quality is inevitable when conventional digital watermark techniques are employed. As another consideration, it is important that digital watermark information be detected reliably from the main information signals, even if the superimposition level is low. That is, it is desired that the probability of erroneous detection of the additional information is as low as possible.
In view of the above, it is an object of the present invention to provide a way to superimpose additional information with an image signal in a manner such that the additional information is inconspicuous on the displayed image, while maintaining the condition that the detection of the additional information can be performed reliably.
It is another object of the present invention to provide an improved digital watermarking method and apparatus.
To achieve the above and other objects, in an illustrative embodiment of the invention there is provided an additional information superimposition apparatus and method in which a dithering process is performed on additional information of a very low level to be added to image information. Prior to the dithering operation, input additional information may be spread out in time via logical operation of an input additional information signal of only a few bits with a pseudo-random code sequence of many bits. In the dithering process, masking patterns are randomly applied to blocks or screens of pixels to mask some of the pixels from superimposition, thereby reducing the effective level of the additional information signal. High frequency random noise is added to the dithered additional information to further reduce the perceptible effect of the additional information on the image. The dithered and noise-added additional information signal is then superimposed with the image signal, and recorded on a storage medium.
The high frequency random noise may be applied to blocks or screens of pixels by randomly selecting among a plurality of random noise patterns of predetermined different noise addition quantities.
In another aspect of the invention, each bit of the additional information is associated with a pixel block region, and individual block regions are divided into sub-block regions. A pattern of sub-block regions is defined for each block region, where the pattern corresponds to the value (one or zero) of the additional information bit associated with that block region. Pixels that are not included in the pattern of sub-block regions are not to be superimposed with the additional information. As a result, the level of the superimposed additional information is further reduced. A dithering process then determines which pixels of the pattern of sub-blocks are to be masked from being superimposed with the additional information.